Habits

When discussing how habits affect out daily activity and happiness I have become very grateful for how I was raised. Sometimes I whole heartily believe something and I have no explanation or reasoning for believing so. This reassured me that I should explore the reasoning behind these feelings and discover if the habits I have formed are true and good. There is one aspect of the theory, however, I do not fully grasp. I do not understand why habits such as biting your nails is considered bad. It is more of a neutral action rather than a bad one. But maybe this in itself shows that it is bad because it does not do good. So is any action that does not immediately create progress toward a good considered a bad habit? Cracking your knuckles, biting your nails, or popping your gum should not be so closely scrutinized as a horrible task.